We report gapless quantum spin liquid behavior in the layered triangular ${\mathrm{Sr}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well separated by Sb planes. Muon spin relaxation measurements show that the $S=\frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature ${\theta }_{\mathrm{CW}}\simeq -143\mathrm{K}$ as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat ($C_m$) below 5 K reveals a $C_m=\gamma T+\alpha T^2$ behavior. The significant $T^2$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $\sim 36\mathrm{K}$ using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($\sim 45\mathrm{K}$) as well as high-temperature susceptibility analysis ($\sim 70\mathrm{K}$). The linear specific heat coefficient is about $18\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ which is somewhat larger than for typical Fermi liquids.

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• Magnetic susceptibility
• Spin dynamics
• Thermal properties
• Strongly correlated systems
• Two-dimensional electron system
• Muon spin relaxation & rotation